Typically, conventional image forming devices including printer and copy machine employ a well-known electrophotographic image forming process. In this image forming process, an image bearing member such as photosensitive drum is electrically charged by a charger. The charged surface of the image bearing member is then exposed to a light, forming an electrostatic latent image. The electrostatic latent image is developed into a visual image which is then transferred onto a sheet such as plain paper.
Among the most common charger used for image forming devices in the art is a corona charger. The corona charger, however, generates a great deal of ozone, leading to a serious environmental disruption and providing an adverse affect for the photosensitive member to reduce duration thereof. For this reason, an alternative image forming process has been expected for years.
To counter this, Japanese Patent Laid-Open Publication No. 1-293358A discloses an image forming device capable of preventing the ozone from being produced. The device, which is illustrated in FIG. 7, includes a photosensitive member generally indicated by reference numeral 101 and an electric-charge bearing member generally indicated by reference numeral 102. The photosensitive member 101 has three layers; a support layer 103, an electrode layer 104 and a photosensitive layer 105, layered in this order. The bearing member 102 has a cylindrical support member 106 mounted for rotation, an electrode layer 107 coated on a periphery surface of the support member 106 and a dielectric layer 108 coated on an entire surface of the electrode layer 107. The photosensitive member 101 and the bearing member 102 are disposed so that the photosensitive layer 105 confronts to the dielectric layer 108 with leaving a small gap therebetween. Also, the photosensitive-member 101 is disposed in a dark.
In operation, a certain voltage is applied between the electrode layers 104 and 107. A light is projected on the photosensitive member 101 in the dark so that it scans in a direction parallel to an axis of the cylindrical bearing member 102. This provides an exposed portion of the photosensitive layer 105 with an electric charge. The electric charge is then discharged to an opposing portion of the dielectric layer 108, forming an electrostatic latent image in the dielectric layer 108. The latent image is then transported by the rotation of the support member 106 into a developing station where it is developed by a developer 109 into a toner image. The toner image is then transported to a transfer station where it is transferred onto a sheet such as paper or film.
The image forming device can certainly prevent the ozone from being produced, though, it has another problems. For example, when reproducing a line image extended in a rotational direction of the cylindrical support member 106 as shown in FIG. 8A, the corresponding latent image formed on the bearing member 102 is unnecessarily extended at a tailing edge thereof as shown in FIG. 8B in which a length of the extended portion is indicated by .DELTA.L. The length of the extended portion increases in proportion to the length of the line image with respect to the rotational direction.
Tests were conducted to determine the causes of the extension. As a result, it was found that the extension might be caused by a residual electric charge or excessive carrier remaining in the photosensitive layer 105 that is continued to be discharged to the dielectric layer 108 even after the completion of the exposure. In the following description, the term "excessive carrier" will be used to mean a carrier or electric charge that is produced at the exposure in the exposed member and moves so slowly so that it is not discharged therefrom.